Japanese Patent Application Laid Open No. 2000-199838 (referred to below as the patent literature 1) proposes, as a prior-art optical module of this type, an optical module formed in a mold package created by resin-molding of a lead frame and a subassembly including a pair of guide pins, an MT ferrule for holding a multi-core optical fiber, and a PD subcarrier for holding a photoreceptor.
FIG. 1A shows the structure of the subassembly of the optical module proposed in the patent literature 1; reference numeral 1 indicates guide pins, reference numeral 2 indicates an MT ferrule, and reference numeral 3 indicates a PD subcarrier. FIG. 1B shows the optical module in which the MT ferrule 2 has been removed from the subassembly.
The MT ferrule 2 includes a combination of a lid 2c and a fiber holding member 2d. There are trapezoidal grooves forming a pair of through holes 2a that house the guide pins 1 in each of the mutually facing surfaces of the lid 2c and the fiber holding member 2d; a plurality of V-shaped grooves 2b for holding optical fibers are formed between the two trapezoidal grooves in the fiber holding member 2d. The lid 2c and the fiber holding member 2d are bonded to each other across the optical fibers. Then, the guide pins 1 are inserted into the through holes 2a. 
The PD subcarrier 3 includes a first plate 3b and a second plate 3c, which form a pair of through holes 3a, and a lead frame, which has heatsink lead pins 3d and connection lead pins 3e as a part and is sandwiched between the first plate 3b and the second plate 3c. The PD subcarrier 3 incorporates a photoreceptor array 3f and a preamplifier IC 3g. 
The MT ferrules 2 precisely positions all optical fibers based on the through holes 2a into which the guide pins 1 are inserted and the PD subcarrier 3 precisely positions all photoreceptors of the photoreceptor array 3f based on the through holes 3a into which the guide pins 1 are inserted. Accordingly, only if the guide pins 1 are inserted into the through holes 2a and 3a, the optical fibers held by the MT ferrule 2 and the photoreceptors of the photoreceptor array 3f incorporated in the PD subcarrier 3 are positioned precisely.
The guide pins 1 project from the MT ferrule 2 as shown in FIG. 1A and are inserted into the corresponding MT connector to which this optical module is connected.
As described above, the guide pins 1 projecting from the MT ferrule 2 are used to make connection with the corresponding MT connector or optical axis alignment. The amount of projection of the guide pins 1 from the MT ferrule 2 and the guide pin hold length in the patent literature 1 are calculated as follows from the standard (JIS C 5981) of the MT connector.
In the JIS C 5981 standard, the guide pin length is defined as 10.8 mm or greater and the length (guide pin hole length into which the guide pin is inserted) in the guide pin insertion direction of the MT ferrule is defined as 8.0 mm. FIG. 2A illustrates the standard; reference numeral 5 shows the guide pins, the reference numeral 6 shows the MT ferrules, and reference numeral 6a shows the guide pin holes. Optical fibers are not shown in this drawing.
Since the guide pin length and the guide pin hole length are defined as show above, in the state where the guide pins 5 mate with the MT ferrule 6, the amount of projection of the guide pins is always 2.8 mm or greater, as shown in FIG. 2B. That is, the guide pin hold length when two MT ferrules 6 are mutually connected via the guide pins 5 is at least 2.8 mm as shown in FIG. 2C. Part of FIGS. 2B and 2C is shown transparently (this is the same as in FIGS. 3A and 3B described later).
Since the guide pins are not secured to any of the MT ferrules, the guide pins can move in the optical axis direction even when the MT connector is connected. Accordingly, the guide pin length is determined so that the guide pin hold length and the amount of projection of the guide pins enough for connection and optical axis alignment can be taken even if the guide pins move more or less.
Similarly, in the optical module in the patent literature 1 that is connected to an MT connector, the guide pins 1 are not secured to the MT ferrule 2. Therefore, the technical common sense of those skilled in the art was that the guide pins 1 should be at least 5.6 mm (2.8 mm+2.8 mm) in length and the MT ferrule 2 should be at least 2.8 mm in length as shown in FIGS. 3A and 3B even when the guide pins 1 and the MT ferrule need to be reduced in length for downsizing of the optical module.